1. Technical Field
The present invention relates to surgical bone-fixation screws. More specifically, the invention relates to interference screws having particular utility in securing a bone plug in a bone tunnel.
2. Discussion of the Prior Art
Interference screws are used in endoscopic surgery where a bone block, bone plug or plastic substitute therefor is to be fixed in a tunnel or other cavity. This is done, for example, in reconstructing the anterior cruciate ligament of the knee. A procedure for reconstructing this ligament is described in U.S. Pat. No. 5,139,520 (Rosenberg), the entire disclosure from which is incorporated herein by reference. A blind tunnel is drilled into the femur and a ligament is inserted into the tunnel, the inserted end of the ligament terminating in a bone plug. To fix the bone plug (and hence the ligament) inside the femur tunnel, an interference screw is driven into the gap between the tunnel wall and the bone plug. The two pieces of bone do not have sufficient clearance between them to accept the screw; accordingly, as the name implies, the screw creates an interference, and it must force or wedge its way into the gap. Once the interference screw is in place, the plug is firmly held in the tunnel, both by the threads of the screw and by friction due to the large force between the tunnel wall and the adjacent side of the plug opposite the interference screw.
Because they are driven into small spaces that are difficult to observe, interference screws have no head, and are driven by Allen wrenches instead of the Phillips or slot-head screw drivers used for most bone screws. The screw usually includes a central cannula or bore allowing it to be slid over a guide wire, previously fed into the tunnel, to keep the screw on track and aligned. The guide wire is slightly smaller in diameter than the cannula.
An interference screw tends to skew and wander off course, especially when it is starting to progress into hard tissue such as bone. The stiffness of the guide wire reduces this tendency but, since the wire is quite thin (on the order of a millimeter), it tends to bend under strong forces and the screw may wander despite riding on the wire. Interference screws are generally thread-forming screws whose helical threads cut into the bone tissue as they advance. If started at the wrong angle when first driven into hard material, the thread-forming interference screw becomes difficult to realign. Since they are inserted endoscopically into a site of limited accessibility and visibility, interference screws are especially apt to be started askew.
A guide wire passed through a cannulated screw is only one technique for keeping the screw properly aligned; other techniques have been used. An example of another technique is found in U.S. Pat. No. 4,927,421 (Goble et al) disclosing an interference screw with an integral drill bit at its forward end. As the screw is turned, the bit cuts a hole equal in diameter to the minor diameter of the following threads. The bit, having drilled a hole, is held therein and prevented from cocking; the threads follow straight on.
Another technique for minimizing skewing is marketed by Acumed, Inc. under the trade name Oregon Fixation System. In that system an interference screw is provided with a transversely small guide tip projecting forwardly from its distal end to facilitate placement and eliminate the need for guide wires. The root of the screw gradually tapers toward the distal end to permit easier starting and provide more gradual compression.
The Goble et al screw, the Oregon Fixation screw and other interference screws heretofore known in the art employ a single helical thread. The use of one helical thread means that the tip of the screw is asymmetrical, since the single thread must start at one particular angular position at the tip and then helically wind along the shank or body of the interference screw from that point. Lateral force develops on the screw thread when it is driven into hard, resisting tissue such as bone. This force tends to cock the screw and turn it away from its intended path, especially when the screw is first started. With a single thread, the screw tip is usually made cone-shaped to minimize lateral force. (A pointed tip spreads out the lateral forces.) Designs with aggressive cutting action, typically non-conical in shape, are impractical with a single thread.
Another drawback to the single thread is that the screw advances slowly. If a certain number of threads per inch are needed for holding power, the advance is limited to the pitch (distance between threads) in a single turn.
The shallow thread angle of the single-thread screw also requires that a high torque be applied to the screw. With steeper angles, a forward force component parallel to the screw axis helps to drive the screw and reduces the required torque. Reduced torque lessens the chance of stripping the screw-driving socket.